Conventionally, antenna devices accommodating a rotatable antenna in a radome are known. JP3123777U discloses this type of antenna device.
The antenna device of JP3123777U is provided with a reflection suppressing plate made of a material having an electrical property similar to that of the radome, inside of the radome at a position on a normal line and separated from the radome by substantially n-fourth (“n” is a positive odd number) of the wavelength of a radio wave. With the structure of JP3123777U, a reflection wave from the radome is canceled by the reflection suppressing plate, and thus an antenna gain reduction and an increase of sidelobes are suppressed.
Although a single antenna transmits and receives radio waves in the antenna device of JP3123777U, antenna devices including a transmission antenna and a reception antenna separated from each other like an FMCW (Frequency Modulated Continuous Wave) radar are also known. In such an antenna device, the transmission and reception of the radio waves are performed simultaneously, and therefore, it becomes important to secure isolation between the transmission and reception antennas.
However, if the radome of JP3123777U is applied to such an antenna device which performs the transmission and reception by the different antennas, the device will have a dual structure having a radome-like structure on the inside of the radome, which causes increases in weight and cost.
Further, it may also be considered to secure the isolation by separating the transmission antenna from the reception antenna in up-and-down directions of the antenna device. However, it will become difficult to reduce in size of the antenna device in the up-and-down directions.
Note that the antenna device of JP3123777U has a slightly inclined side wall of the radome. With an antenna device in which transmission and reception antennas are arranged in the up-and-down directions, by providing the inclination angle to the side wall of the radome as above, it is considered to be capable to improve the isolation characteristic to some extent. However, in order to secure a sufficiently high isolation characteristic, the side wall of the radome needs to be inclined sharply to some extent, which causes an increase in radome diameter.